1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile device and a plotter which comprise an optical writer for writing a latent image by irradiating a laser beam onto the surface to be scanned of an image carrier such as a photosensitive drum, and in which the latent image formed on the image carrier is developed using a developer and then successively transferred to a transfer material and fixed by fixing means.
2. Description of the Related Art
Conventional optical writers of this type are apt to be affected by dust and, for this reason, the housing thereof in which optical components such as a polygon mirror are housed has high airtightness, the assembly thereof being performed in a clean room.
Japanese Laid-Open Patent Application No. 2001-066540 discloses an optical writer of a configuration for switching the direction of rotation of a mirror between when a latent image is written on an image carrier and when it is not. In this optical writer, negative pressure is produced on the section of the reflective surface of the mirror toward the rotating direction side from the center thereof and, even if dust and dirt attaches to this section, because the mirror can be rotated in the reverse direction to when a latent image is being written, the dust and dirt attached to the reflective surface to that point can be separated from the reflective surface by exposure to a fast flow of air, whereupon sticking of dirt and dust to the mirror can be prevented without need for a structure in which the mirror is airtight-sealed to be adopted.
However, because of the floating toner, paper dust and dust and so on present in the interior of, for example, a laser printer in which an optical writer is commonly mounted, the penetration of at least a minute amount of dust and dirt into the housing is inevitable. As described in detail below, dust and dirt that penetrates the housing attaches to each mirror surface of the polygon mirror from which the optical writer is configured, and the attachment of this dust has a significant effect from the viewpoint of reducing the reflectance thereof. Especially, since mirror surface reflectance differs between the rotation direction end and the opposing direction end thereto of a polygon mirror, a so-called shading correction in which the optical intensity of the laser is changed in accordance with the position in the scan direction is performed, but there is a problem in that, because the shading changes occur frequently, the correction value thereof must be changed just as frequently which, for the operator or serviceman of this optical writer, is a troublesome and inefficient operation.
Technologies relating to the present invention are also disclosed in, for example:
Japanese Laid-Open Patent Application No. 2002-162585
Japanese Laid-Open Patent Application No. 2003-295095
Japanese Laid-Open Patent Application No. 2003-329959
Japanese Laid-Open Patent Application No. 2004-054116
Japanese Patent No. 3652238